If you need to know the original concentration of your sample (i.e. counting bacteria or absolute counts from any gated population), you will love the concentration calculator feature of FCS Express. Volume can be determined by three methods: comparing with a known amount of reference beads, using time and flow rate, or using the Volume ($VOL) keyword.
You can even have the concentration shown directly on the statistics window of any plot. When a gate is moved on your plot the concentration statistics will update automatically as well.
In the example below, the concentration calculator is being used to calculate the concentration of the cells in Region 2. The sample was spiked with 5x10^5 reference beads that are in Region 1.